1. Field Of The Invention
This invention relates to television channel combiners and, more particularly, to arrangements for combining adjacent television channels onto a single transmission line.
2. Description Of The Prior Art
Normal broadcast television signals are a combination of a vestigial side-band, amplitude modulated, bandwidth limited picture or visual signal and a separate, frequency modulated sound or aural signal. The combined visual and aural signals, i.e., the television signal, occupy a particular frequency band which is referred to as a television channel. The frequencies occupied by the visual and aural signals are unique for each separate channel so that the channels remain distinct and maintain their separate information contained therein when the signals are converted in a standard television receiver. Television channels can also be defined as preassigned frequency ranges assigned to the visual and aural signals for a particular transmission. In the usual definition of a range of television channels, the highest frequency assigned to one channel is identical to the lowest frequency assigned to the immediately higher and adjacent channel. Similarly, the lowest frequency assigned to a particular channel is identical to the highest frequency assigned to the immediately lower and adjacent channel.
While commercial television stations often broadcast only a single channel through the air to a user's home, it is also common for cable companies or the like to transmit a plurality of television channels over the air to subscribers. Such wireless cable operations include Multichannel Multipoint Distribution Service (MMDS) systems as well as Instructional Television Fixed Service (ITFS) systems. In one known system operating in the microwave range, thirty-one (31) separate television channels are provided in adjacent six MHz increments, beginning at a frequency of 2500 MHz. In other words, television channel "One" occupies the frequency range of 2500-2506 MHz, channel "Two" occupies the frequency range of 2506-2512 MHz, and so on. Rather than utilize thirty-one (31) separate antennas for each channel, it is common to combine the channels to reduce the amount of transmission equipment used to transmit all channels to subscribers.
In one known arrangement, shown in FIG. 1, alternate television channels are combined in a first system, using the frequency ranges of the omitted channels between each combined channel as protective guard bands. The remaining alternate or non-adjacent television channels are combined in a separate, second system, once again using the frequency ranges of the omitted channels, which are transmitted in the first system, as guard bands. Referring to FIG. 1, a first transmitter 2 for television channel "1", which includes separate video and audio signals, has its output signal coupled to a first non-adjacent channel combiner 4. Similarly, a third transmitter 6 for television channel "3", which likewise includes separate video and audio signals, has its output signal coupled to the first non-adjacent channel combiner 4. Although not shown in FIG. 1, any remaining odd-numbered television channels would also have their output signals coupled to the first non-adjacent channel combiner 4, where all odd-numbered television channels are combined together into a first composite signal. The first composite signal from the first non-adjacent channel combiner 4 is coupled to a first transmission line 8. For a system operating in the microwave frequency range, a length of waveguide would be used for this first transmission line 8. The first transmission line 8 carries the first composite signal from the first combiner 4 to a first transmitting antenna 10. Here, the first composite signal is transmitted through the air to a receiving antenna 12 feeding standard receiving equipment 14.
Similarly, a second transmitter 16 for television channel "2" has its video and audio output signals coupled to a second non-adjacent channel combiner 18 and a fourth transmitter 20 for television channel "4" has its video and audio output signals coupled to tee second non-adjacent channel combiner 18. Any remaining even-numbered television channels would also have their output signals coupled to the second non-adjacent channel combiner 18 where all even-numbered television channels are combined together to form a second composite signal. A second transmission line 22 carries the second composite signal from the second combiner 18 to a second transmitting antenna 24, where the second composite signal is transmitted through the air to the receiving antenna 12.
The output signals from the various channel transmitters, for a microwave system, would be coupled to the channel combiners by multi-section directional filters or the like. FIG. 2 shows a typical response for one channel of a three section filter used in coupling the visual and aural signals for that channel to a non-adjacent channel combiner. The response includes most of the usable information in the six MHz frequency range of the channel itself, otherwise referred to as the occupied channel. The response begins to drop off in the upper and lower adjacent channels and significantly drops off to negligible levels in the first upper non-adjacent channel and the first lower non-adjacent channel. FIG. 3 shows the typical response of a non-adjacent four channel combiner which uses the omitted channels as guard bands to separate the channels from each other.
The FIG. 1 arrangement requires two separate channel combiners, two separate waveguide transmission lines and two separate transmitting antennas to transmit the plurality of television channels to a single receiving antenna 12. This duplication of equipment substantially increases the cost of the overall system, particularly when very expensive waveguides are used for the transmission lines in systems operating in the microwave range of frequencies.
There exists in the art a need for channel combiners which can combine immediately adjacent television channels, if only to reduce this expensive duplication of transmission equipment.
A number of prior art adjacent channel combiners have been proposed to avoid the duplication inherent in dual transmission line systems. These include the arrangements shown in U.S. Pat. Nos. 2,531,447; 4,029,902; 4,258,435; 4,815,075 and 4,839,894. Other prior art known to the applicants which is relevant to this field include U.S. Pat. Nos. 2,816,270; 3,428,918; 3,453,638; 3,461,407; 3,710,281; 3,806,838; 3,865,990; 3,971,922; 4,652,879 and 4,847,574. These prior art arrangements employ complex hybrid combining methods that are either expensive or difficult to build or maintain, or introduce signal loss, or both.
In order to overcome the disadvantages of the prior art, it is an object of the present invention to provide an arrangement for combining a plurality of adjacent television channels onto a single transmission line without increasing hardware requirements and without serious degradation of signal parameters or power.